Image credit: Henniker Scientific
Polydimethylsiloxane (PDMS) is a component of a group of polymeric organosilicon compounds that are generally known as silicones.
It is the most extensively used of these compounds and is featured in applications ranging from cosmetics and medicines to silly putty. This article focuses on the use of PDMS in microfluidic device fabrication.
Figure 1. Molecular structure of PDMS. Image credit: Henniker Scientific
A number of properties which make PDMS favorable for use in creating microfluidic devices are listed below:
- Transparent (240 nm – 1100 nm range)
- Low auto florescence
- Few nm resolution molding
- Low cost
One major drawback of PDMS when used in this application is its poor adhesion to glass, which results in premature device failure.
The Henniker HPT-200 system is specifically designed and optimized to produce consistent plasma treatment performance for repeatable and reliable bonding of PDMS. Both the PDMS substrates and glass are treated with air plasmas, at low pressure, with all settings under microprocessor control.
Image credit: Henniker Scientific
On both substrates, the treatment is successful at eliminating hydrocarbon groups (CxHy) leaving behind OH groups on the glass substrate and silanol groups on the PDMS, respectively. This enables strong Si – O – Si covalent bonds to form between the two materials through the process as shown in Figure 2.
Figure 2. Schematic diagram of the plasma treatment process to improve surface adhesion. Image credit: Henniker Scientific
Contact Angle Measurements
Contact angle measurements indicate whether a surface is hydrophilic (under 90°) or hydrophobic (over 90°) or by the angle a water drop makes with the surface. In this case, it is clear that if the time or power of an air plasma treatment on PDMS is increased, then it results in a more hydrophilic surface.
Figure 3. Contact angle variations with increasing treatment time at 25% power (left) and increasing power with 10secs exposures (left). Both show a switch between hydrophobic to hydrophilic behavior. Insets show example droplets. Image credit: Henniker Scientific
This switch to more hydrophilic behavior indicates that the proposed treatment has been effective and that the – OH termination coupled with silanol groups is now exposed, leading to better bonding of the PDMS to the glass substrates.
X-Ray photoelectron spectroscopy (XPS)
XPS is a technique that is increasingly being used to analyze the functional groups present on the substrate surface. Here, XPS is employed to demonstrate how C-O groups are absent before plasma treatment (in this case with oxygen plasma) and present after the treatment, representing an effective surface modification.
Figure 4. XPS results from untreated (Top) and oxygen treated (Bottom) samples highlighting the presence of C-O groups after treatment. Image credit: Henniker Scientific
Bonding of PDMS to glass is a one of the major concerns in the use of the material in fabricating microfluidic devices.
This issue has been addressed by employing a Henniker model HPT-200 bench top plasma system to treat PDMS substrates along with glass substrates. It is shown that the wettability of both surfaces have been improved with plasma treatment. This in turn leads to increased bonding between the two substrates which is emphasized in Figure 6. In addition, the system can also be optimized to bond PDMS to thermoplastic materials.
Figure 5. PDMS microfluidic channels undergoing plasma treatment. Image credit: Henniker Scientific
Figure 6. Bonded PDMS and glass following a plasma treatment in a Henniker HPT-200 machine. Image credit: Henniker Scientific
This information has been sourced, reviewed and adapted from materials provided by Henniker Plasma.
For more information on this source, please visit Henniker Plasma.